This invention relates to permanent magnets and more particularly to anisotropic permanent magnets of manganese-aluminum-carbon (Mn-Al-C) alloys.
Previously known Mn-Al alloy magnets consisting of Mn 60-75 weight % (hereinafter referred to simply as %) and the remainder aluminum are such that the ferromagnetic metastable phase (face-centered tetragonal, lattice constant a = 3.94A, c = 3.58A, c/a = 0.908 and a Curie point of 350.degree. to 400.degree.C; hereinafter referred to as the .tau. phase) is obtained by way of a heat treatment such as by the cooling control method or the quenching-tempering method. The ferromagnetic .tau. phase is the metastable phase which appears between the high temperature phase (close-packed hexagonal, lattice constant a = 2.69A, c = 4.38A; hereinafter referred to as the .epsilon. phase) and the room temperature phase (a phase in which the alloy is separated into the AlMn(.gamma.) phase and the .beta.-Mn phase). This intermediate phase was discovered by Nagasaki, Kono, and Hirone in 1955. (Digest of the Tenth Annual Conference of the Physical Society of Japan, Vol. 3, 162, October, 1955.)
However, the above Mn-Al alloys possess magnetic characteristics which are low, i.e. in the order of (BH)max = 0.5 .times. 10.sup.6 G.Oe, Br = 2200 G, and .sub.B Hc = 600 Oe. Since then, a method has been developed of sintering the powdered alloy in the .tau. phase whereby the coercive force is increased by pulverizing; however, the magnetic characteristics of these alloys in isotropic form, at best, were low, being in the order of (BH)max = 0.6 .times. 10.sup.6 G.Oe, Br = 1700 G, and .sub.B Hc = 1250 Oe. Moreover, since the products were formed from powder, their mechanical strengths were low, which makes these products impractical for commercial use.
On the other hand, a method has been proposed for improving the magnetic characteristics of these Mn-Al alloy magnets by applying a high degree of cold-working on the alloy in the .tau. phase (ferromagnetic phase) to render them anisotropic. It is known that rod shaped Mn-Al magnets in the .tau. phase are sealed in nonmagnetic stainless steel pipes, and while being held in said pipes are subjected to cold-working, such as swagining, to a degree of 85-95%. This method is capable of producing an anisotropic permanent magnet possessing magnetic characteristics in the order of Br = 4280G, .sub.B HC = 2700 Oe, and (BH)max .apprxeq. 3.5 .times. 10.sup.6 G.Oe in the direction of preferred magnetization, i.e., the axial direction of the rod. Because Mn-Al alloy magnets are intermetallic compounds having very hard and brittle mechanical properties, however, even a cold-working of less than 1% causes cracks or fractures in the alloys.
On the other hand, since the degree of anisotropization is dependent upon the degree of cold-working, it is necessary to cold-work the alloy to a high degree, normally higher than 80%, in order to achieve satisfactory magnetic characteristics, and in order to be able to conduct such cold-working step, the cold-workig operation must be conducted while the alloy is sealed in a nonmagnetic stainless steel pipe.
An anisotropic permanent magnetic obtained by using the above method is complicated in that the Mn-Al alloy inside the pipe must be finely pulverized into powder, and, moreover, it is difficult to obtain rods of uniform cross-section. The method is therefor costly and of little practical value.
In order to overcome the above difficulties, a method has been proposed of obtaining a rod shaped anisotropic Mn-Al alloy magnet by subjecting the .tau. phase of the Mn-Al alloy magnet to hydrostatic extrusion at a temperature below 200.degree.C, but the magnetic characteristic of such alloys is low, being in the order of (BH)max = 2.5-3.6 .times. 10.sup.6 G.Oe in the direction of preferred magnetization. This method also requires a very intricate hydrostatic extrusion operation and is again a very impractical method.
To replace the Mn-Al alloy magnets mentioned above, there have been invented manganese-aluminum-carbon alloy magnets in bulk shape having excellent magnetically isotropic characteristics, which magnets were disclosed in U.S. Pat. No. 3,661,567. Thus, according to U.S. Pat. No. 3,661,567, the Mn-Al-C alloy magnets may be obtained as isotropic permanent magnets in bulk shape excelling in magnetic characteristics, stability, weathering resistance and mechanical strength. These alloys may be mult-component alloys containing impurities or additives other than Mn, Al and C, but should contain Mn, Al, and C as indispensable component elements, with the component ratio of Mn, Al, and C in these multi-component alloys falling within the following range:
Mn 69.5.about.73.0% Al 26.4.about.29.5% C 0.6.about.(1/3 Mn-22.2)%
which alloys are manufactured under the restricted conditions described hereinunder:
Thus, Mn. Al and C are so mixed that each component falls within the respective composition range mentioned above, then the mixture is heated to a temperature higher than 1,380.degree.C but lower than 1,500.degree.C, in order to obtain a homogeneous melt with carbon forcibly dissolved therein, and thereafter the molten alloy is cast in a suitable mold. The ingot thus-obtained is heated above 900.degree.C to form its high temperature phase, and then, is quenched by rapidly cooling it from a temperature above 900.degree.C to a temperature below 600.degree.C at a cooling rate of higher than 300.degree.C/min. The quenched alloy is then tempered by heating it at a temperature of 480.degree.-650.degree.C. for an appropriate period of time. A Mn-Al-C alloy magnet in bulk shape obtained in this way has magnetic characteristics better than (BH)max = 1.0 .times. 10.sup.6 G.Oe, while in an isotropic state. This magnetic characteristic runs twice as high as the magnetic characteristics of isotropic Mn-Al alloy magnets.
The Mn-Al-C alloy magnets obtained in this way were isotropic in their bulk state, with the (BH)max running higher than 1.0 .times. 10.sup.6 G.Oe, and their mechanical strengths were as follows: hardness H.sub.RC = 45, tensile strength = 1-2 kg/mm.sup.2, elongation = 0, compressive strength = 100 kg/mm.sup.2, and transverse strength = 7 kg/mm.sup.2.
The Mn-Al-C alloy magnets had serious disadvantages, however, in that in the course of trying to further improve their magnetic characteristics; by whichever method of the above mentioned cold working method or the powder forming method, the magnetis characteristics may be barely improved or rather degraded, and any improvement in their performance by way of anisotropization could not be anticipated.